Varying degrees of reduced susceptibility to dental caries can be imparted by altering the chemical composition of the apatite incorporated in tooth enamel. The effects of fluoride in this respect are well-known. Because apatite is subject to a great number of chemical substitutions, its chemical composition is quite variable. Many of these substitutions can either directly or indirectly affect the caries resistance of dental enamel. A single-crystal X-ray diffraction study of synthetic and naturally occurring calcium apatites that contain tetrahedral substituents for phosphorus will be carried out. Detailed structural and physical parameters will be evaluated in order to determine how the various tetrahedral species are accommodated in the apatite crystal structure, and their effects on apatite crystal chemistry, particularly with regard to possible mechanisms of caries resistance.